The Silicon Controlled Rectifier
(one type of thyristor) is a
four layer device, PNPN from anode
(A) to cathode (here labeled "C",
sometimes also labeled "K"). An SCR is like a junction
transistor with a fourth layer and therefore a total of
three P-N junctions.
Meanwhile, a third terminal, the gate (G), makes an SCR
function like an odd hybrid of transistor
and diode:

The two outer junctions
are forward biased by the voltage as shown, but the inner
junction is reverse biased.
As a result, a small current in
the gate electrode can turn on a current
between anode and cathode.
SCRs are latching devices; once an SCR is turned on, or
"fired," it remains on (i.e., the current
will continue to flow) until the driving voltage between
anode and cathode
is removed. The minimum anode -
cathodecurrent
required to keep the SCR on (once triggered) is called the
holding current,
IH.

An SCR is called a rectifier because (much like a
"vanilla" diode) it conducts
current in only direction. It is
a unidirectional thyristor, in
contrast to a triac. You can think
of an SCR as being a conventional (diode)
rectifier, with a gate controlling forward resistance.
Additionally, an SCR can be switched on (in the absense of a
gate voltage) by applying enough forward voltage to overcome
its internal resistance. This is normally considered a
design limitation, though, and switching is normally
controlled via gate voltage.

Knowing all this, an SCR's characteristic V - I curve
then looks like this:

SCRs are not often used in BEAM
-- they are tough to find since they're special-purpose
devices in electronics (used for lighting control, motor
speed control and other variable power applications). In
combination with a UJT or PUT
they can, though, make a mean solar
engine.

In a pinch, you can build up something very like an SCR
using discrete transistors
wired as a complementary feedback pair: